The Teachers' Health Study, a large medical experiment involving 18,000 female teachers, attempted to determine whether aspirin could help prevent heart attacks. In this study, one group of about 9,000 teachers took an aspirin every other day, while a control group took a placebo. After several years, it was determined that the teachers in the group that took aspirin had significantly fewer heart attacks than the teachers in the control group. Which of the following statements explains why it would not be appropriate to say that everyone should take an aspirin every other day? I. The study included only teachers, and different results may occur in individuals in other occupations.

II. The study included only females and there may be different results with males.

III. Although taking aspirin may be helpful in preventing heart attacks, it may be harmful to some other aspects of health.
A. I only
B. II only
C. II and III only
D. I and III only
E. I, II, and III